Composition for and method of producing phosphate films on metals



March 22, 1949. B. v. M BRIDE 2,465,247

COMPOSITION FOR AND METHOD OF PRODUCING PHOSPHATE FILMS ON METALS FiledOct. 24, 1946 Fly].

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I I I 16 v4 WITNESSES: INVENTOR Patented Mar. 22, 1949 COMPOSITION FORAND METHOD OF PRO- DUCING PHOSPHATE FILMS ON METALS Byron V. McBride,Irwin, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application October 24,1946, Serial No. 705,373

4 Claims.

This invention relates to the treatment of ferrous, zinc and cadmiummetal surfaces by means of adherent paste-like compositions to produceprotective phosphate coatings on predetermined portions of the metalsurfaces.

Heretofore, in the art of producing protective phosphate coatings onmetal surfaces, it has been quite difiicult to treat only predeterminedportions of the metal surfaces. Phosphate compositions, as known in theart today, are so fluent that regardless of their method of application,they can not be confined to predetermined sources unless the metalsurfaces which it is desired not to treat are masked off. Spraying ofthe phosphate compositions on the metal surfaces results in running anddripping of the phosphate composition indiscriminately and no adequatecontrol of application confining the solution to certain areas ispossible. Dipping or other modes of application of the relatively fluidphosphate compositions obviously will result in all the surfaces beingsubjected to the action of the phosphate coatings. For many purposes, itis necessary that the protective phosphate compositions be applied toonly predetermined portions of the metal surfaces without running,dripping or other contamination of the rest of the surfaces of theparticular member being treated. No phosphate composition known has beenfound suitable or adequate for controlled treatment of metal surfaces.

The object of the present invention is to provide a paste-like adherentcoating composition which may be applied to predetermined portions ofsurfaces of members to produce thereon protective phosphate filmswithout undesirable action on other portions of the surfaces.

A further object of this invention is to produce a paste-like adherentcoating composition which, when applied to metal surfaces, will producethereon protective phosphate films and a readily removable powderedlayeron drying.

A still further object of the invention is to provide for treating metalsurfaces to produce thereon protective phosphate films by applyingpastelike coating compositions.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description anddrawing, in which:

Figure 1 is a vertical cross section through a member treated inaccordance with the invention; and

Fig. 2 is likewise a vertical cross section showing a further step inthe process of treatment,

In accordance with the invention, paste-like compositions that arerelatively permanently stable have been produced for application toferrous, zinc and cadmium metal surfaces whereby to produce protectivephosphate coatings therein at any predetermined portion of the surface.After-treatment of the metal surfaces is easy and highly protectivephosphate films on the metal surface are obtained. The paste-likecoating compositions are produced by combining a finely divided poroussilica aerogel with an aqueous solution capable of producing phosphatefilms when applied to the metal surfaces. Outstanding results have beenobtained when from 10 to 45 parts by weight of a porous silica aerogel,finely divided to the extent that at least about 50% passes through a180 mesh screen, is combined with 100 parts by weight of an aqueousphosphate solution. The aqueous phosphate solution should containessentially from 25% to by weight of a phosphorous compound capable ofsupplying phosphate ions to react with the metal and an oxidizing agent,the balance of the solution being water. The composition is relativelypaste-like so that when applied by means of brushing or even byspraying, the composition remains at the point of application to themetal as an adherent coating that does not flow or drip. Furthermore,the paste-like nature of the composition is relatively permanent and itmay be stored for prolonged periods of time without undue separation andit may be employed without continual stirring. Other advantages of thecomposition will be set forth hereinafter.

The porous silica aerogel may be produced in various ways, thecomposition disclosed in the Patent 2,093,454 to Kistler or the productsold to the trade as Santocel 45 or Santocel 58 are suitable for use inthe present invention. The porous silica aerogels have a dry bulkcondensity of from 6 to 9 pounds per cubic foot. Their pH varies from4.5 to 6.5. The silica aerogels are composed of from to of SiOz, thebalance being volatile matter and impurities such as alkali sulphates.It has been found that the best pastelike compositions of the presentinvention are secured if at least 50% of the silica aerogel passesthrough a mesh screen.

The porous silica aerogel has been found to possess new and unexpectedproperties enabling the production of paste-like compositions entirelydifferent from those obtainable by the use of other finely divided inertinorganic materials.

For example, tests have shown that diatomaceous earth will not form apaste. When combined with a phosphate solution, the diatomaceous earthwill settle out in a few seconds. Fuller's earth will not form a pasteand it also settles out in a few seconds. Finely powdered ceramic fritsdo not make a good paste and when combined with the phosphate solutionhad been found to be detrimental to the production of a satisfactoryphosphate coating on ferrous metal surfaces. Finely divided titaniumdioxide similarly results in a poor, spotty phosphate film on the metalsurfaces. A further defect encountered with compositions embodying manyof the finely divided inorganic powders other than silica aerogel hasbeen that such compositions when applied to metal surfaces and driedproduce a strongly adherent powder layer, which is undesirable since itrequires excessive scrubbing and brushing to remove, the powderedflayer. Thus bentonite, when combined with a phosphate solution, formslayers extremely adherent to the metal surfaces on drying and theremoval of the bentonite powder entails a considerable amount of handscrubbing. For instance, a stream of compressed air at30 lbs. per squareinch pressure would not remove any of these coatings. By contrast, theporous silica aerogel compositions of this invention produce smoothprotective phosphate films onthe metal and on drying a loose fluffylayer of aerogel is present that may be removed by a simple rinsing inwater, a light brushing with a brush or cloth, or a stream of compressedair.

Corrosion tests in salt spray and steam chests show that protectivephosphate films produced byapplications of the silica aerogelcompositions of this invention are superior to those secured with othercompositions embodying ceramic frit, infusorial earth, asbestine,calcium carbonate and the like.

. .Examples of the paste-like compositions of the present invention are:

Example I 1 /2 pounds of the porous silica aerogel sold to thetrade asNo. 58 micropulverized Santocel is admixed with one gallon of acomposition composed of the following ingredients.

Phosphoric acid 84% pounds 5 Manganese phosphate do 1% Sodium nitrate doA Cupric nitrate ounces 4 Water to makeone gallon The compositionresulting on admixture of the ingredients was a smooth paste that couldbe readily brushed upon metal surfaces or applied by'means of a spraygun. The composition adhered to the metal surfaces without running.After {application to the metal surfaces, the coating air dried in about15 minutes and produced asmooth uniform protective phosphate film on themetal surface and an upper layer of loose and fluffy silica aerogel. Thelayer of silica aerogelcould be removed by brushing with a softbristle'brush or dipping in water or directing against it a spray ofwater or by a blast of compressed air.

' Ea'ample II "'14i parts by weight of a porous silica aerogel wasadmixed with an aqueous solution having the following composition:'

Parts Zinc dihydrogen phosphate 14 Copper nitrate Nitric acid 1 Water 49Methylcellulose (water soluble) 1 Wetting agent /g Orthophosphoric acidto give a pH of from 1 to 2.

The wetting agent was a sodium alkyl aryl sulfonate sold to the tradeunder the name Nacoonal. The composition was a smooth, thick paste thatcould be applied to steel panels as a thin uniform coating that adheredto the surfaces to which it was applied. The composition formedprotective phosphate films in a few minutes and on air drying a fluffy,nonadherent layer of silica aerogel resulted.

"Steam chest and salt spray tests indicate that the phosphate film isconsiderably more corrosion resistant than the film secured by use ofthe same phosphate solution without the silica aerogel.

' Paste-like compositions were prepared with as little as 10 parts byweight of the silica aerogel to parts by weight of the aqueous phosphatesolution. The paste was readily applied by spraying. Thicker pastes thatcould be readily applied by brushing or spraying were prepared with asmuch as 45 parts by weight of the silica aerogel to the liquid phosphatesolution.

The use of a water soluble cellulose ether in the proportion of one-halfto 5% of the weight of the phosphate solution improved the stability ofthe paste. Suitable cellulose ethers are water soluble methyl celluloseand ethyl cellulose formulations.

A small amount of wetting agent enables the prompt admixture of thesilica aerogel in aqueous phosphate solution. A wetting agent in theamount of from 0.1% to 3% of the weight of the phosphate solution isbeneficial. Suitable wetting agents are the sulfonates and sulfates oflong carbon chain organic compounds.

The phosphate solutions may contain free phosphoric acid and acidicphosphate compounds. The pH of the phosphate solutions will vary from0.5 to 6. The oxidizing agent may be a nitrate, a nitrite or othercompound capable of oxidizing hydrogen which results from the reactionof the metal and the phosphate solution.

Referring to Fig. 1 of the drawing, there is illustrateda metal sheet l2whose upper surface I4 is to be treated to provide a protectivephosphate coating thereon. The paste-like silica aerogel composition ofthe present invention is applied as a coating I6 to the upper surface ofthe member I 2. For most purposes a coating of a thickness of inch to 13' of an inch in thickness is adequate. At room temperature, aprotective phosphate film will be produced on the surface I4 in a matterof a few minutes. If desired, the treated metal surfacecan be put in anoven in order that the coating react more promptly and the silicaaerogel form a dried layer. Even surfaces that are slightly oily can becoated with the composition and a satisfactory ,phosphate film produced.I

A single application of the composition has been found to produceheavier and more uniform phosphate films thanthe aqueous phosphatesolution without the silica aerogel would be capable of producing.Vertical and overhead surfaces can be easily treated without running ordripping occurring. The phosphate films produced on 5. such vertical oroverhead surfaces are equal in quality to that produced on upperhorizontal surfaces.

It is believed that the porous silica aerogal retains the phosphatesolution in such a manner that it is released to the metal surfaceduring the chemical reaction With the metal at a uniform rate over theentire surface. Apparently, none of the phosphate solution is retainedwithin the body of the porous silica aerogel and the silica aerogeldries readily. After the reaction of the phosphate solution with themetal surface has been completed and any water present has evaporatedinto the atmosphere, the silica aerogel is not bonded appreciably to thephosphate film and is present as a light fiufiy layer that may beremoved with a single stroke of a soft bristle brush, by dipping inwater or with a stream of compressed air. No scrubbing or other involvedmanual labor is necessary to remove the layer of dried silica aerogel.

In order to seal off the protective phosphate coating and enhance itsprotective properties, it is desirable to apply thereto water solublehexavalent chromium compound such as chromic acid or sodium dichromate.A paste-like chromium composition may be prepared by admixing 100 partsby weight of the silica aerogel with from to 200 parts by weight of thehexavalent chromium compound and sufiicient water to produce a fiowablepaste. The chromium composition so produced may be applied to thephosphate film in the manner shown in Fig. 2 of the drawing. The metalsurface M of the member l2 carrying the protective phosphate film I8 iscovered with a coating 20 of the paste-like chromium containingcomposition. The coating 28 may be dried in about minutes and a simplebrushing or dipping in water will remove the dried silica aerogel whichforms and is in a loose powdery state. The inhibitive effect of thechromium derived from layer has been found to be far superior to thatsecured with the conventional dilute chromic acid rinse common in theart today. A suitable hexavalent chromium composition is as follows:

Example III Porous silica aerogel oz 13 Sodium dichromate oz 13 Watergal- 1 The composition forms a smooth paste that may be brushed on andit will not drip or spread undesirably.

The protective phosphate films produced by means of the paste-likecoating composition of the present invention are so superior to thoseproduced by other compositions that it is desirable to employ thecompositions even when treating all the surfaces of a metal member. Theuse of the paste-like com-position is also advantageous in cases whereonly a small amount of metal is being treated since expensiveinstallations, such as dip tanks, conveyors and the like, areunnecessary.

Since certain changes may be made in the above invention and differentembodiments of the invention may be made without departing from thescope thereof, it is intended that all matter contained in the abovedisclosure shall be interpreted as illustrative and not in a limitingsense.

I claim as my invention:

1. A stable paste-like coating composition suitable for use in producingprotective phosphate coatings on ferrous, zinc and cadmium metalsurfaces composed of from 10 to 45 parts by weight of a porous silicaaerogel finely divided to the extent that at least-about 50% passesthrough a mesh screen, and 100 parts by weight of an aqueous "phosphatesolution having a pH of from 0.5 to 6, the phosphate solution composedessentially of from 25% to 70% by weight of a phosphorus compoundcapable of supplying phosphate ions to react with the metal and anoxidizing agent, and the balance of the phosphate solution being water,the porous silica aerogel rendering the paste-like nature of thecomposition relatively permanent, the composition being sufficientlyfiowable to be applied as a thin smooth coating to the metal surfaceswhich upon drying forms a relatively nonadherent silica aerogel and asmooth underlying protective phosphate film on the metal.

2. A stable paste-lik coating composition suitable for use in producingprotective phosphate coatings on ferrous, zinc and cadmium metalsurf-aces, composed of from 10 to 45 parts by weight of a porous silicaaerogel finely divided to the extent that at least about 50% passesthrough a 100 mesh screen, from 0.1 to 3 parts by weight of a wettingagent, from to 5 parts by weight of a water soluble cellulose ether, and100 parts by weight of an aqueous phosphate solution having a pH of from0.5 to 6, the phosphate solution composed essentially of from 25% to 70%by weight of a phosphorus compound capable of supplying phosphate ionsto react with the metal and an oxidizing agent, and the balance of thephosphate solution being water, the porous silica aerogel rendering thepaste-like nature of the composition relatively permanent, thecomposition being sufficiently flowable to be applied as a thin, smoothcoating to the metal surfaces which upon drying forms a relativelynonadherent silica aerogel layer and a smooth underlying protectivephosphate film on the metal.

3. In the process of producing protective phosphate coatings on ferrous,zinc and cadmium metal surfaces the steps comprising applying apaste-like layer of a coating composition composed essentially of from10 to 45 parts by weight of a porous silica aerogel finely divided tothe extent that at least about 50% passes through a 100 mesh screen, and100 parts by weight of an aqueous phosphate solution having a pH of from0.5 to 6, the phosphate solution composed essentially of from 25% to 70%by weight of a .phosphorus compound capable of supplying phosphate ionsto react with the metal, and an oxidizing agent, and the balance of thephosphate solution being water, drying the coating, removing therelatively nonadherent layer of silica aerogel produced on drying, andexposing a smooth protective phosphate film on the metal surfaces.

4. In the process of producing protective phosphate coatings on ferrous,zinc and cadmium metal surfaces the steps comprising applying apaste-like layer of a coating composition composed essentially of from10 to 5 parts by weight of a porous silica aerogel finely divided 'tothe extent that at least about 50% passes through a 100 mesh screen, and100 parts by weight of an aqueous phosphate solution having a pH of from0.5 to 6, the phosphate solution composed essentially of from 25% to 70%by weight of a phosphorus compound capable of supplying phosphate ionsto react with the metal, and an oxidizing agent,

7 and thebalance of the phosphate solution bein water, drying thecoating,removing the relative ly rionadhererit layer 'of silica aerogelproduced on drying, exposing -a smooth protective 'phosphate film on themetal surfaces, applying to the phosphate film a, paste-like coating ofa second composition composed of 100 parts by weight of the silicaaerogel and from 10 to 200 parts by weight of a water soluble hexavalentchromium compound and sufiicient water to produce a flowable paste,drying the applied coating of the second com-position whereby arelatively nonadherent layer of silica aerogel is produced, and removingthe last-mentioned layer of silica aerogel.

BYRON V. MCBRIDE.

8; REFERENCES CITED The following references are of record inthe file ofthis patent: V

UNITED STATES PATENTS Number Tanner Nov. 1'7, 1942

